With regard to pipes for transporting a medium like water and gas, steel pipes are conventionally employed as such. While steel pipes exhibit a high internal pressure resistance strength and an excellent creep resistance, they are poorly quakeproof and develop rust and corrosion. In these days, therefore, synthetic resin pipes such as unplasticized polyvinyl chloride (PVC) pipes and polyethylene pipes are frequently employed.
A method for producing synthetic resin pipes is disclosed, for example, in Japanese Patent Application Laid-open No. H10-225988. A hollow article made of a crystalline thermoplastic resin is drawn by a clamp, from between a die and a former whose circumferential side surface is formed with a plurality of ridges having a section with a curvature radius of 0.5 mm or greater. Thereby, the pipe is formed as stretched in the axial and circumferential directions.
Synthetic resin pipes are exceptionally remarkable in terms of quakeproof property and impact resistance. On the other hand, they show a limited internal pressure resistance strength and a poor creep resistance. When a pipe element like a pipe or a hose is employed to transport various substances including liquid, gas, etc. in a flowing manner, the pipe element requires sufficient pressure resistance so as not to break under the pressure by an internal flow of substances. In particular, high internal pressure resistance is necessary for hydraulic oil piping, drain pipes and the like for carrying high-pressure fluids. From this point of view, Japanese Patent Application Laid-open No. H8-11250, for example, discloses a composite pipe which comprises a tubular inner layer and a tubular outer layer each made of a synthetic resin or other flexible materials, and which further includes a fiber reinforcing layer and a wire reinforcing layer interposed therebetween.
In this composite pipe, although the fiber reinforcing layer and the wire reinforcing layer improve the pressure resistance, they also cause following problems.
First of all, the resin reinforcing layer is prepared by braiding fibers of suitable thickness or by winding such fibers in spiral form. This process increases the thickness of the resin reinforcing layer by itself. Besides, in the resin reinforcing layer comprising fiber bundles, the strength in the circumferential direction which corresponds to the longitudinal direction of each fiber can be much greater than the strength in the axial direction. Further, because the wire reinforcing layer is laminated on the resin reinforcing layer, this structure increases the overall wall thickness and total weight, and thereby sacrifices the handlability and economical efficiency.
Secondly, when used to transport substances in a flowing manner, such composite pipes may fail to provide stable long-term service, depending on the physical properties and state of the substances to be transported. By way of example, when used as a hot-water pipe, a steampipe or like pipe for ships, or a chemical transport pipe or like pipe at chemical works, etc., an inner layer made of an ordinary synthetic resin may melt because of hot water, drug solution, etc., or deteriorate through chemical reactions. Furthermore, when used for transporting sand, ore and the like, an inner layer made of an ordinary synthetic resin may wear out and break only after a short service.
In addition, current demands include recycling of raw materials and associated economic effects. However, due to the complexity in separating the resin reinforcing layer and the wire reinforcing layer, recycling is a problematic task.
For the purpose of solving the above problems found in conventional technologies, the first object of the present invention is to provide high-pressure composite pipes which show an excellent pressure resistance and optimum applicability to various use, and also to provide lightweight high-pressure composite pipes which can be produced economically.
In another aspect, according to the conventional technologies for joining high-pressure composite pipes, they cannot be joined to each other directly, if the materials for the inner layer and the outer layer are different from those for the resin reinforcing layer and the wire reinforcing layer. Hence, such pipes have been joined by way of a pipe joint or the like. This technique has presented additional problems in terms of the strength, sealing property, etc. at the connection of the pipe joint and the high-pressure composite pipes. In summary, even if a high-pressure composite pipe has an excellent pressure resistance, any trouble in pipe arrangement can cause leakage or rupture at the connection of the high-pressure composite pipes.
For the pipe arrangement which includes a butt-fused connection area between the high-pressure composite pipes, attempts have been made to prevent damage at the butt-fused area under a load of internal pressure. For example, Japanese Patent Application Laid-open No. H11-101383 describes a method for bonding the exterior of the butt-fused area with a reactive resin. According to this method, after the butt fusion, a mold is mounted on the fused area, and a reactive resin is poured into the gap between the pipe and the mold. The resin is allowed to solidify for reinforcement.
However, as far as the reactive resin bonds the outer layers only, the outer layers can peel from the reinforcing layers. In this case, reinforcement is ineffective at the joint area of butt-fused high-pressure composite pipes. Besides, this method is troublesome from the viewpoint of construction, not only because the reactive resin should be handled with care but also because the mold should be removed after the resin has solidified. Further, since the solidified reactive resin is hard but brittle, it cracks easily when the high-pressure composite pipes are flattened.
With an intention of solving these problems, the second object of the present invention is to provide a method for joining high-pressure composite pipes, which method is capable of firmly joining high-pressure composite pipes having an excellent pressure resistance by a simple operation, firmly joining the high-pressure composite pipes by reinforcing the joint strength at the butt-fused area, and making a joint area adjustable to strain of the high-pressure composite pipes.